It has been desired over the decades to overcome the problems associated with resource saving and environment pollution on the global scale. Apart from the nuclear power, development efforts were made on the technology of effectively utilizing the potential energy in wind, tide and sunlight as the replacement energy for fossil fuel, with some being commercially implemented.
Among others, a focus is put on solar cells as the main technology for the utilization of clean sunlight energy. Due to its capability of low cost, simple, and small-scale power generation, the solar power generation technology has been commercially utilized in houses and buildings to provide a partial replacement of the energy consumption therein.
In the current mainstream solar cell system for residential houses, a plurality of panel-like solar cell modules are connected in series or parallel and arrayed and installed on the roof so that the desired power may be produced. The solar cells used in the panel-like solar cell modules are shaped square or pseudo-square with cut corners so as to conform to the shape of the solar cell panel.
The solar cell is a semiconductor device for converting light energy into electricity and includes p-n junction type, pin type and Schottky type, with the p-n junction type being on widespread use. When classified in terms of substrate material, the solar cell is generally classified into three categories, crystalline silicon solar cells, amorphous silicon solar cells, and compound semiconductor solar cells. The crystalline silicon solar cells are sub-divided into monocrystalline and polycrystalline solar cells. The crystalline silicon solar cells become most widespread since crystalline substrates for solar cells can be relatively easily manufactured.
In general crystalline silicon solar cells, a p-n junction must be formed to separate carriers created by irradiation of sunlight. In one example where the substrate used is p-type silicon, an n-type silicon layer is formed on the light-receiving surface by diffusing a Group V element such as phosphorus. In another example where the substrate used is n-type silicon, a p-type silicon layer is formed on the light-receiving surface by diffusing a Group III element such as boron.
The silicon solar cell is manufactured from a p-type silicon substrate, for example, by thermal diffusion of a dopant like phosphorus at a temperature of about 800 to 950° C. to form diffusion layers on both the entire surfaces of the substrate. If desired, an unnecessary portion of the diffusion layer is removed, and the remaining layer serves as the diffusion layer in the solar cell.
Then an antireflective coating, for example, silicon nitride film is formed on the diffusion layer. Silver paste in grid pattern on the light-receiving surface and aluminum paste on the substantially entire back surface are printed and fired to form electrodes, yielding a crystalline silicon solar cell.
From the standpoint of increasing the photovoltaic conversion of a solar cell, a thinner diffusion layer is better. However, too thin a thickness leads to a likelihood of breakage of n-type layer by the electrode, known as punch-through, and current collection at the electrode is inhibited due to an increased resistance. Therefore, the structure known as “selective emitter” is used in which the diffusion layer is thin in the light-receiving surface area as a high resistance layer (low concentration diffusion) and thick in the electrode area as a low resistance layer (high concentration diffusion).
The selective emitter may be prepared by covering the surface of a substrate with an anti-diffusion coating such as SiO2, removing lines of the anti-diffusion coating to open diffusion windows, and selectively diffusing a dopant into the window region to form a high-concentration diffusion layer.
After the high-concentration diffusion layer is formed, the anti-diffusion coating is removed, and the dopant is diffused into the entire surface including the high-concentration diffusion layer, so that the region surrounding the high-concentration diffusion layer may become a low-concentration diffusion layer having a lower dopant concentration than the high-concentration diffusion layer.
Next, an antireflective coating is formed on the surface. The ARC may be a silicon nitride, titanium oxide or aluminum oxide film. Such a film may be formed by CVD, for example.
The silicon oxide, silicon nitride, titanium oxide and aluminum oxide films used as the ARC all serve to terminate defects on the silicon wafer surface and improve the properties, especially short-circuit current of solar cells.
After the ARC is formed, electrode fingers are formed by printing. The position of electrode fingers is determined by registering the substrate in place using its two sides as the reference, and aligning the fingers with the high-concentration diffusion layer.
After the diffusion treatment, the low-concentration diffusion layer and the high-concentration diffusion layer cannot be discriminated by visual observation. This discrimination is possible by electrical evaluation, for example, measurement of spreading resistance, which is a destructive, time-consuming test.
Since the solar cell manufacture process involves many steps as mentioned above, there arises a problem that if the substrate is rotated in a certain step, then the orientation of the substrate can no longer be identified. To avoid such inconvenience, the substrate may be provided with a mark by laser marking, but the mark is difficultly ascertained after an ARC is formed thereon. An additional problem of laser marking is that the solar substrate is distorted thereby, with properties of the relevant portion being degraded.
The prior art references pertinent to the present invention include the following documents.